The bible simply does not teach that all men have sinned without exception. Of course it does! ALL have sinned. NO person is sinless but the God/man Jesus Christ. Some people are WORSE sinners than others, but everyone sins. If ANY person was without sin, he/she could have saved Jesus the trouble and died for all the sinners instead of him. Are there some people not conscious of their sin like children or the mentally impaired? Yes, and we trust the grace of God to deal with them in His love and mercy. But, to say the Bible doesn't teach that all mankind has sinned is simply false. Here are just a few places this is taught:
But the scripture has consigned all under sin, that the promise by faith in Jesus Christ might be given to them that believe. (Galatians 3:22)
What shall we conclude then? Do we have any advantage? Not at all! For we have already made the charge that Jews and Gentiles alike are all under the power of sin. (Romans 3:9)
For God has bound everyone over to disobedience so that he may have mercy on them all. (Romans 11:32)
Indeed, there is no one on earth who is righteous, no one who does what is right and never sins. (Eccles. 7:20)
If we say that we have no sin, we deceive ourselves, and the truth is not in us. (I John 1:8)
Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned-- (Romans 5:12)
Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son. This is the verdict: Light has come into the world, but people loved darkness instead of light because their deeds were evil. Everyone who does evil hates the light, and will not come into the light for fear that their deeds will be exposed. But whoever lives by the truth comes into the light, so that it may be seen plainly that what they have done has been done in the sight of God. (John 3:18-21)